Large quantities of inexpensive silicon are required for use in the photovoltaic field. Apart from economic viability, the suitability of the material is conditional, most importantly, on its compliance with purity requirements. Although the permitted residual contamination concentrations in solar silicon can lie clearly above those required of silicon in the semi-conductor industry, it is not possible at present to refine contaminated silicon to the required degree of purity in a single stage. This is due to the variety of contaminating elements and the concentrations thereof. Metallic impuritis, in particular alumium and iron are predominant. However, boron and phosphorus have a particular undesirable effect due to their doping character. In addition, removal of oxygen and carbon also poses a particular problem.
Metallic impurities can be removed by treatment of the silicon with acids, but it is not possible to reduce the residual concentrations to the desired low values below 0.1 ppmg by this method.
As described in U.S. Pat. No. 2,402,839, a purification effect can also be achieved by vacuum treatment of the melt. However, this is not suitable for removing relatively non-volatile impurities such as boron and carbon.
In German Pat. No. 3,504,723, on the other hand, the carbon and oxygen content is reduced in molten silicon which has been obtained from a charge comprising silicon compounds and carbon in an electric arc furnance by hydrogen gasification.
However, this process is not capable of removing residual metal, boron or phosphorus contents.
It is therefore an object of the present invention to provide a process according to which impure silicon can be brought to a stage of purity which complies with the requirements in the photovoltaic field from an economic point of view.